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Haloalkanes and Haloarenes 1010Unit Objectives HaloalkanesHaloalkanes andand After studying this Unit, you will be able to HaloarHaloarHaloarHaloarenesenesenesenesenesenes • name haloalkanes and haloarenes according to the IUPAC system of nomenclature from their given structures; Halogenated compounds persist in the environment due to their • describe the reactions involved in resistance to breakdown by soil bacteria. the preparation of haloalkanes and haloarenes and understand various reactions that they The replacement of hydrogen atom(s) in an aliphatic undergo; or aromatic hydrocarbon by halogen atom(s) results • correlate the structures of in the formation of alkyl halide (haloalkane) and aryl haloalkanes and haloarenes with halide (haloarene), respectively. Haloalkanes contain various types of reactions; halogen atom(s) attached to the sp3 hybridised carbon • use stereochemistry as a tool for atom of an alkyl group whereas haloarenes contain understanding the reaction halogen atom(s) attached to sp2 hybridised carbon mechanism; atom(s) of an aryl group. Many halogen containing • appreciate the applications of organic compounds occur in nature and some of organo-metallic compounds; these are clinically useful. These classes of compounds • highlight the environmental effects of polyhalogen compounds. find wide applications in industry as well as in day- to-day life. They are used as solvents for relatively non-polar compounds and as starting materials for the synthesis of wide range of organic compounds. Chlorine containing antibiotic, chloramphenicol, produced by microorganisms is very effective for the treatment of typhoid fever. Our body produces iodine containing hormone, thyroxine, the deficiency of which causes a disease called goiter. Synthetic halogen compounds, viz. chloroquine is used for the treatment of malaria; halothane is used as an anaesthetic during surgery. Certain fully fluorinated compounds are being considered as potential blood substitutes in surgery. In this Unit, you will study the important methods of preparation, physical and chemical properties and uses of organohalogen compounds. 2021–22 10.110.110.1 Classification Haloalkanes and haloarenes may be classified as follows: 10.1.1 On the These may be classified as mono, di, or polyhalogen (tri-,tetra-, etc.) Basis of compounds depending on whether they contain one, two or more halogen Number of atoms in their structures. For example, Halogen Atoms Monohalocompounds may further be classified according to the hybridisation of the carbon atom to which the halogen is bonded, as discussed below. 10.1.2 Compounds This class includes Containing (a) Alkyl halides or haloalkanes (R—X) sp3 C—X Bond (X= F, In alkyl halides, the halogen atom is bonded to an alkyl group (R). They form a homologous series represented by C H X. They are Cl, Br, I) n 2n+1 further classified as primary, secondary or tertiary according to the nature of carbon to which halogen is attached. If halogen is attached to a primary carbon atom in an alkyl halide, the alkyl halide is called primary alkyl halide or 1° alkyl halide. Similarly, if halogen is attached to secondary or tertiary carbon atom, the alkyl halide is called secondary alkyl halide (2°) and tertiary (3°) alkyl halide, respectively. (b) Allylic halides These are the compounds in which the halogen atom is bonded to an sp3-hybridised carbon atom adjacent to carbon-carbon double bond (C=C) i.e. to an allylic carbon. Allylic carbon Allylic carbon (c) Benzylic halides These are the compounds in which the halogen atom is bonded to an sp3-hybridised carbon atom attached to an aromatic ring. Chemistry 290 2021–22 10.1.3 Compounds This class includes: Containing (a) Vinylic halides sp2 C—X Bond These are the compounds in which the halogen atom is bonded to a sp2-hybridised carbon atom of a carbon-carbon double bond (C = C). (b) Aryl halides These are the compounds in which the halogen atom is directly bonded to the sp2-hybridised carbon atom of an aromatic ring. 10.2 Nomenclature Having learnt the classification of halogenated compounds, let us now learn how these are named. The common names of alkyl halides are derived by naming the alkyl group followed by the name of halide. In the IUPAC system of nomenclature, alkyl halides are named as halosubstituted hydrocarbons. For mono halogen substituted derivatives of benzene, common and IUPAC names are the same. For dihalogen derivatives, the prefixes o-, m-, p- are used in common system but in IUPAC system, as you have learnt in Class XI, Unit 12, the numerals 1,2; 1,3 and 1,4 are used. The dihaloalkanes having the same type of halogen atoms are named as alkylidene or alkylene dihalides. The dihalo-compounds having both the halogen atoms are further classified as geminal halides or gem-dihalides when both the halogen atoms are present on the same carbon atom of the 291 Haloalkanes and Haloarenes 2021–22 chain and vicinal halides or vic-dihalides when halogen atoms are present on adjacent carbon atoms. In common name system, gem-dihalides are named as alkylidene halides and vic-dihalides are named as alkylene dihalides. In IUPAC system, they are named as dihaloalkanes. Some common examples of halocompounds are mentioned in Table 10.1. Table 10.1: Common and IUPAC Names of some Halides Structure Common name IUPAC name CH3CH2CH(Cl)CH3 sec-Butyl chloride 2-Chlorobutane (CH3)3CCH2Br neo-Pentyl bromide 1-Bromo-2,2-dimethylpropane (CH3)3CBr tert-Butyl bromide 2-Bromo-2-methylpropane CH2 = CHCl Vinyl chloride Chloroethene CH2 = CHCH2Br Allyl bromide 3-Bromopropene o-Chlorotoluene 1-Chloro-2-methylbenzene or 2-Chlorotoluene Benzyl chloride Chlorophenylmethane CH2Cl2 Methylene chloride Dichloromethane CHCl3 Chloroform Trichloromethane CHBr3 Bromoform Tribromomethane CCl4 Carbon tetrachloride Tetrachloromethane CH3CH2CH2F n-Propyl fluoride 1-Fluoropropane Example 10.1 Draw the structures of all the eight structural isomers that have the molecular formula C5H11Br. Name each isomer according to IUPAC system and classify them as primary, secondary or tertiary bromide. o SolutionSolutionSolution CH3CH2CH2CH2CH2Br 1-Bromopentane (1 ) o CH3CH2CH2CH(Br)CH3 2-Bromopentane(2 ) o CH3CH2CH(Br)CH2CH3 3-Bromopentane (2 ) o (CH3)2CHCH2CH2Br 1-Bromo-3-methylbutane (1 ) Chemistry 292 2021–22 o (CH3)2CHCHBrCH3 2-Bromo-3-methylbutane(2 ) o (CH3)2CBrCH2CH3 2-Bromo-2-methylbutane (3 ) o CH3CH2CH(CH3)CH2Br 1-Bromo-2-methylbutane(1 ) o (CH3)3CCH2Br 1-Bromo-2,2-dimethylpropane (1 ) Write IUPAC names of the following: Example 10.2 (i) 4-Bromopent-2-ene (ii) 3-Bromo-2-methylbut-1-ene SolutionSolutionSolution (iii) 4-Bromo-3-methylpent-2-ene (iv) 1-Bromo-2-methylbut-2-ene (v) 1-Bromobut-2-ene (vi) 3-Bromo-2-methylpropene Intext Question 10.1 Write structures of the following compounds: (i) 2-Chloro-3-methylpentane (ii) 1-Chloro-4-ethylcyclohexane (iii) 4-tert. Butyl-3-iodoheptane (iv) 1,4-Dibromobut-2-ene (v) 1-Bromo-4-sec. butyl-2-methylbenzene. 10.310.310.3 Nature ofofNature Halogen atoms are more electronegative than carbon, therefore, carbon-halogen bond of alkyl halide is polarised; the carbon atom bears C-X BondBondC-X a partial positive charge whereas the halogen atom bears a partial negative charge. As we go down the group in the periodic table, the size of halogen atom increases. Fluorine atom is the smallest and iodine atom is the largest. Consequently the carbon-halogen bond length also increases from C—F to C—I. Some typical bond lengths, bond enthalpies and dipole moments are given in Table 10.2. Alkyl halides are best prepared from alcohols, which are easily accessible. 293 Haloalkanes and Haloarenes 2021–22 Table 10.2: Carbon-Halogen (C—X) Bond Lengths, Bond Enthalpies and Dipole Moments Bond Bond length/pm C-X Bond enthalpies/ kJmol-1 Dipole moment/Debye CH3–F 139 452 1.847 CH3– Cl 178 351 1.860 CH3–Br 193 293 1.830 CH3–I 214 234 1.636 10.410.410.4 Methods ofofMethods The hydroxyl group of an alcohol is replaced by halogen on reaction with PreparationPreparationPreparation concentrated halogen acids, phosphorus halides or thionyl chloride. of Haloalkanes Thionyl chloride is preferred because in this reaction alkyl halide is formed along with gases SO2 and HCl. The two gaseous products are escapable, hence, the reaction gives pure alkyl halides. The reactions of primary and 10.4.1 From Alcohols secondary alcohols with HCl require the presence of a catalyst, ZnCl2. With tertiary alcohols, the reaction is conducted by simply shaking the alcohol with concentrated HCl at room temperature. Constant boiling with HBr (48%) is used for preparing alkyl bromide. Good yields of R—I may be obtained by heating alcohols with sodium or potassium iodide in 95% orthophosphoric acid. The order of reactivity of alcohols with a given haloacid is 3°>2°>1°. Phosphorus tribromide and triiodide are usually generated in situ (produced in the reaction mixture) by the reaction of red phosphorus with bromine and iodine respectively. The preparation of alkyl chloride is carried out either by passing dry hydrogen chloride gas through a solution of alcohol or by heating a mixture of alcohol and concentrated aqueous halogen acid. The above methods are not applicable for the preparation of aryl halides because the carbon-oxygen bond in phenols has a partial double bond character and is difficult to break being stronger than a single bond (Unit 11, Class XI). 10.4.2 From (I) From alkanes by free radical halogenation Hydrocarbons Free radical chlorination or bromination of alkanes gives a complex mixture of isomeric mono- and polyhaloalkanes, which is difficult to Chemistry 294 2021–22 separate as pure compounds. Consequently, the yield of any single compound is low (Unit 13, Class XI). (II) From alkenes (i) Addition of hydrogen halides: An alkene is converted to corresponding alkyl halide by reaction with hydrogen chloride, hydrogen bromide or hydrogen iodide. Propene yields two products, however only one predominates as per Markovnikov’s rule.
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